1. Field of the Invention
The present invention relates to blow-molded multi-layer plastic containers having excellent surface luster. More specifically, the invention relates to plastic containers of a multi-layer structure having outer surfaces of an olefin resin yet exhibiting excellent surface luster.
2. Description of the Prior Art
Plastic hollow containers obtained by blow-molding are light in weight, resistant against shocks, and have been widely used as packaging containers for a variety of foods, condiments, toiletries and the like. The containers of this type have been produced in a variety of types having a single layer and multiple layers. From the standpoint of sanitation and waterproof property, polyolefin resins such as polyethylene, polypropylene, etc. have been widely used for forming the inner and outer surfaces of the containers.
Polyolefin resins for hollow-molding are usually of a grade different from those for ordinary molding. That is, during the hollow-molding, there easily takes place phenomenon (called draw-down) in which the molten parison hangs down due to its own weight or a phenomenon (melt fracture) in which the molten parison itself undergoes undulation or exhibits sharkskined appearance. To prevent the occurrence of such defects, use has been made of a resin having a molecular weight which is relatively at high as that the melt flow rate (MFR) is 2.0 g/10 min. or smaller and that the molecular weight distribution as defined by Mw/Mn is as wide as from 3.5 to 7. This resin may be effective in preventing the draw-down but is not effective in preventing the melt fracture. When the containers are obtained by using this resin, furthermore, the container walls lack surface luster and transparency. Moreover, the products packaged in the bottles made of this resin appear to be considerably inferior to the products packaged in the glass bottles, depriving the products of commercial values.
Japanese Laid-Open Patent Publication No. 215529/1990 discloses a multi-layer plastic container of a structure having two or more resin layers that are laminated one upon the other and further having excellent surface luster, wherein the outermost surface layer is composed of an ethylene-propylene random copolymer having an ethylene content of 2 to 8% by weight, a melt flow rate of 2.5 g/10 min. or greater and an Mw/Mn ratio of 3 to 5.
In producing olefin resin bottles, so far, it is necessary to use a sand-blasted metal-mold cavity in order to prevent the resin from being melt-adhered onto the metal mold surfaces and to extract the air during the molding. In practice, however, the sand-blasted surface is transferred to the outer surfaces of the olefin resin bottles that are molded and deteriorates the luster of the outer surfaces of the bottles.
In the above-proposed blow-molded multi-layer containers, the surface luster is surely improved compared with the general olefin resin bottles but is not still satisfactory compared with other molded articles such as films and the like.